Maytansine is a recently discovered ansa macrolide with potent anti-mitotic activity. It is currently being tested as an anti-tumor chemotherapeutic agent. It is known to prevent microtubule assembly. We have found that maytansine operates by a hitherto unknown and unique mechanism of action, in that is induces rapid microtubule disassembly and enhances the alkylation of tubulin. We intend to elucidate this mechanism of action by various methods. The binding of (3H)maytansine to tubulin the microtubule subunit protein, will be measured and the results will be compared with those for vinblastine, which is a competitive inhibitor of maytansine binding to tubulin. The conformational effects of maytansine on the tubulin molecule will be analyzed by different probes. Experiments will be done to see which end of the microtubule maytansine attacks and the mechanism of this attack will be examined. The hypothesis put forward on the basis of presently available evidence is that maytasine is the exogenous analogue of the calcium-dependent regulatory protein (CDR). Consequently, the effects of CDR on tubulin conformation, maytansine binding and microtubule disassembly will also be studied. Finally, since maytansine enhances alkylation (as opposed to other anti-tubulin drugs like vinblastine, which suppress alkylation), it is likely that maytansine, in combination with an alkylating agent, will have a synergistic effect. Thus, the effect on micotubule assembly and disassembly in vitro of maytansine in combination with iodacetamide, diamide, and chlorambucil will be examined. Similarly, maytansine in combination with alkylating agents will be tested for its ability to block mitosis in cultured cells and fertilized sea urchin eggs. It is possible that these last experiments may lead to the development of a method for using maytansine as part of a combination chemotherapy for the treatment of cancer.